The present invention relates to a heat fin assembly (hereunder sometimes referred to merely as a "fin" or "heat sink fin") for use as a heat sink and to a process for producing the heat sink fin. More particularly, the present invention relates to a heat sink fin produced by means of a multi-wire saw.
The heat sink fin according to the present invention can be used in various applications, but in particular it will be described as a heat sink for IC packages in which an IC chip is airtightly sealed.
FIGS. 1A and 1B are respectively a planar view and a cross-sectional view of a ceramic IC package, which is one type of IC package with which the present invention can be employed.
The IC chip generally indicated by 1 is placed within a airtight space 4 that is defined by a ceramic base 2 and a ceramic cap 3. The chip 1 is fixed in a central cavity of the base 2. The base 2 and the cap 3 are sealed together by means of a glass layer 6, with leads 5 being held in the glass layer 6. The leads are connected to the IC chip 1 via lead wires 7. In fabricating the package, the materials forming the base 2, cap 3, leads 5 and glass layer 6 are selected such that they will have generally the same coefficient of linear expansion, and the utmost care is exercised to insure reliability for the package.
As the integration of IC increases, a need has arisen to insure that the heat generated by the IC chip 1 is dissipated to the outside of the package. Otherwise, the heat would cause erroneous IC operations or the air-tightness of the package would be damaged to cause serious troubles. One possible way to avoid this problem is to make the base 2 in FIG. 1B of a good heat conductor. However, the ability of such a structure to dissipate heat is limited, since heat is dissipated only from the surface of the base 2.
Under the circumstances, there has been proposed an IC package of such a structure that heat generated from the IC can be efficiently released to the outside of the package.
FIG. 2 shows an example of this structure as applied to a ceramic package of a pin grid array type. An IC chip generally indicated by 1 is fixed at the center of a heat conducting plate 12 and placed within an airtight space 14 that is defined by frame-like ceramic plates 9 and 10 and a lid 13. The metallic lid 13 and the ceramic plate 9, as well as the heat conducting plate 12 and the ceramic plate 10, are joined together by a suitable method such as brazing, and the mating surfaces of the ceramic plates 9 and 10 are sealed together by means of a glass layer. Pins 11 inserted into the ceramic plate 9 are connected electrically to the circuit on the IC chip 1 via lead wires 7 and the conductive circuit drawn on the surface 10a of the ceramic plate 10.
The material forming the heat conducting plate 12 is selected from among those materials which have a high heat conductivity but which have a small difference in linear expansion coefficient from the chip 1, such as copper-impregnated tungsten or Kovar (trade name of Co-Ni-Fe alloy). A heat sink fin 8 is joined to the underside of the heat conducting plate 12.
The fin 8 is composed of both a bottom plate 8b to be mounted on the heat conducting plate 12 and a plurality of heat dissipating portions 8a. Heat generated from the IC chip 1 is transmitted through the heat conducting plate 12 to reach the bottom plate 8b and heat dissipating portions 8a, where it is dissipated from the fin surfaces 8d. To promote the dissipation of heat from the fin surfaces 8d, a cooling medium in either gaseous or liquid form is permitted to flow in the heat dissipating portions 8a. Thus, the fin 8 must satisfy certain conditions both as to heat conduction and heat transfer characteristics.
To improve the heat conduction characteristics of the heat sink fin 8, it is necessary that a material of high heat conductivity such as aluminum or copper be used and that the heat dissipating portions 8a have a large total cross-sectional area. To improve the heat transfer characteristics of the heat sink fin 8, it is necessary that the pressure loss when the cooling medium passes through the heat dissipating portions 8a be minimized and that the heat dissipating portions 8a have a large total surface area. With a view to meeting these requirements, heat sink fin of various shapes have heretofore been proposed.
FIGS. 3A and 3B show typical examples of a prior art fin.
FIG. 3A shows a channel-type fin 8', that has plate-shaped heat dissipating portions 8a' (hereunder referred to as "heat dissipating plates") arranged parallel to one another. This type of heat sink fin assembly is hereunder referred to as a "channel fin". FIG. 3B shows a pin-type fin 8" that has pin-shaped heat dissipating portions 8a" (hereunder referred to as "heat dissipating pins") erected in an array. This type of heat sink fin assembly is hereunder referred to as a "pin fin".
The difference between the two types of fins lies in the manner in which the cooling medium flows in the heat dissipating portions 8a' or 8a". In the channel fin of FIG. 3A, the cooling medium will flow in only one direction along the heat dissipating plates 8a', whereas the pin fin 8" of FIG. 3B enables the cooling medium to flow in every direction. Further, the presence of many surfaces on which the cooling medium will impinge contributes to promote cooling by virtue of what is generally referred to as the "boundary layer renewal effect".
The channel fin 8' is generally produced by either a plastic working method or by a machining method.
FIG. 4 is an illustration of a hot extrusion method used to manufacture the channel fin assembly 8' of FIG. 3A. A heated billet 16 is inserted in the container 15 and is pressed with a plunger 17 so that a long semifinished product 19 is extruded through a hole 20 of a die 18 that is of the same shape as the hatched area A of FIG. 3A. A prescribed length of the extrudate is cut off to produce the channel fin 8'. When producing the channel fin 8' by extrusion, the thickness t of and gaps g between heat dissipating plates 8a' shown in FIG. 3A are limited. The lower limit of the thickness t is said to be about 2 mm, below which working by extrusion becomes very difficult. The lower limit for the gaps g is said to be about 3 mm, below which the strength of die hole 20 cannot be insured. For the same reason, the height h of the heat dissipating plates 8a' is generally so set that it will not exceed about 5 times the gap g.
When producing the channel fin 8' by machining, milling is adopted. FIG. 5 illustrates how milling is performed. A predetermined number of milling cutters 21 are rotated over a workpiece 22 (a material block of the same outside dimensions as the channel fin assembly 8' to be produced) to cut grooves 23, which are made progressively deeper by continuing the revolution of milling cutters 21. The thickness t of and gaps g between the heat dissipating plates 8a' shown in FIG. 3A are also limited when producing the channel fin 8' by milling. The lower limit of the thickness t is said to be about 1.5 mm in order to prevent the walls defining each groove 23 from being fractured by the cutting force acting on the walls. The lower limit for practical purposes of the gaps g is said to be about 1.5 mm, below which the rigidity of milling cutters 21 cannot be insured. For the same reason, the height h of the heating dissipating plates 8a' is generally so set that it will not exceed about 10 times the gap g.
As discussed above, the channel fin 8' produced by the conventional plastic working or machining method has a large plate thickness t and gap g between heat dissipating plates 8a', and their height h is also limited Hence, if one wants to assure the necessary surface and cross-sectional areas by providing a predetermined number of heat dissipating plates 8a', the planar dimensions W.sub.1 .times.W.sub.2 of the channel fin 8' will inevitably increase.
There are a number of ways of producing the pin fin 8", including machining and die casting. However, all of these methods have such poor production efficiency that it is not economically feasible to adopt them in large-scale production. As an example, machining with a milling machine will be described below.
As shown in FIG. 5, the process starts by cutting shallow grooves in one direction in one surface of a workpiece 22. Then, the workpiece 22 is rotated 90.degree. in the same plane and grooves are cut in such a direction that they cross the previously formed grooves at right angles. This procedure is repeated so that the depth of the grooves will increase progressively to eventually yield the pin fin 8" shown in FIG. 3B. The grooves are made progressively deeper in order to insure that the walls of grooves formed in the first stage of working will not be deformed by the cutting force of the milling cutters 21. The working method under discussion involves certain dimensional limitations due to the need to prevent deformation from occurring during the cutting operation. The lower limits on the thickness d.sub.1 (d.sub.2) of the heat dissipating pins 8a" and the gaps g.sub.1 (g.sub.2) therebetween are said to be about 2 mm and 2.5 mm, respectively, whereas the height h of the pins is generally so set that it will not exceed about 10 times the value of g.sub.1 (g.sub.2).
Thus, the pin fin assembly 8" produced by the machining method has large thicknesses d.sub.1 (d.sub.2) and gaps g.sub.1 (g.sub.2) and the height h is also limited. Hence, if one wants to assure the necessary surface and cross-sectional areas by providing a predetermined number of heat dissipating pins 8a", the planar dimensions W.sub.1 .times.W.sub.2 of the pin fin 8" will inevitably increase.
Increasing the planar dimensions W.sub.1 .times.W.sub.2 will naturally increase the size of the electronic device in which the package is installed. Hence, this is not a feasible approach.